Americans spend an estimated $1.3 billion on fish oil products every year, making them one of the most commonly consumed dietary supplements in the country.

But do you know what’s in your fish oil?

A new documentary, “Supplements and Safety,” pulls back the curtain on some of America’s most popular supplements, and it suggests that many people who buy them may not be getting what they are paying for. The program, airing on the PBS investigative series “Frontline” on Tuesday night, is a collaboration between “Frontline,” The New York Times and The Canadian Broadcasting Corporation.

The program examines the widespread use of potent vitamins, herbs, fish oil and fat-burning supplements. Millions of Americans use these products safely every year. But researchers have found that in many cases they can cause unexpected side effects. And because dietary supplements are largely unregulated by the federal government, adulteration and contamination are common, experts say.

The Frontline documentary investigates large outbreaks of disease tied to tainted vitamins and fat-burning supplements, including one case in which a workout supplement was linked to more than 70 cases of liver damage. The company whose products were at the center of that outbreak, USPlabs, is among 117 companies and individuals that the Justice Department filed criminal and civil enforcement actions against last year.

The “Frontline” program follows doctors, patients, industry experts and regulators to uncover the root of these outbreaks. It looks at whether the widespread use of vitamin D and other popular vitamins that are commonly sold and taken in high doses can do more harm than good. One that it focuses on is fish oil, the third most popular supplement in America, which is used by at least one in 10 adults.

Despite their popularity, some studies have found that roughly three-quarters of fish oil supplements on the market do not contain the amount of omega-3 fatty acids advertised on their labels. Some have also found that fish oil supplements are prone to becoming rancid.

“I think for cardiovascular disease, one has to say that there is no compelling evidence that taking fish oils protects against a first heart attack or a second heart attack,” Andrew Grey, the author of the JAMA study and an associate professor of medicine at the University of Auckland, says in the “Frontline” report. “So people who are advised to do that or are doing it are wasting their time and their money.”

Proponents of fish oil supplements argue that omega-3 fatty acids are essential for heart function and promote overall health. Experts say there is no doubt about that. But there is no clinical evidence that taking high doses in supplement form is beneficial for most people.

To learn more about the debate over fish oil, vitamins and herbal supplements, watch “Supplements and Safety” at 10 p.m. Eastern on PBS stations on Tuesday, Jan. 19.